This invention relates, in general, to the manufacture of carbon fiber-reinforced composite materials. More particularly, it pertains to a process for the surface treatment of carbon fiber by electrolytic oxidation so as to improve the adhesion of the fiber to a matrix in the manufacture of carbon fiber-reinforced composite materials. The invention is effectively applicable to the surface treatment of carbon fibers made of not only polyacrylonitrile (PAN) and pitchy materials but also other materials as precursors.
In recent years there has been a growing demand for composite materials reinforced with carbon fiber and which exhibit great strength. One of the most important considerations in the manufacture is improved adhesion between the matrix material and the carbon fiber. The adhesion is known to be strikingly improved by an oxidation treatment of the carbon fiber surface, and various ways of surface treatment have hitherto been suggested.
The present invention is concerned, in particular, with a process for the surface treatment of carbon fiber based on the so-called electrolytic oxidation process that involves anodic oxidation of the carbon fiber by continuous supply of a direct current to the fiber as the positive electrode.
For the electrolytic oxidation, applying a uniform surface treatment to the carbon fiber being fed is essential. To attain this end, for example, U.S. Pat. No. 4,234,398 teaches varying the relative distance between the running filaments and the cathode plate in an electrolytic cell so as to keep the density of current passing across the fiber surface constant throughout the length of the fiber. In the case of Japanese Patent Application Public Disclosure No. 132126/1983, uniform electrolysis is aimed at by flowing an electrolyte countercurrently over moving filaments and thereby preventing the deposition of evolving gas upon the filament surface.
These processes have been found ineffective, however, when a carbon fiber tow comprising a number of filaments is to be treated for electrolytic oxidation. Our investigations in this connection have revealed that, with tows consisting of from 1,000 to 24,000 filaments, for example, the progress of oxidation differs between the central portion and the peripheral portion of each tow. The oxidation progresses little in the center but to excess peripherally. Consequently, the filaments in the center of the tow are not sufficiently surface treated. When such a carbon fiber tow is used in the manufacture of a carbon fiber-reinforced composite material, there is no appreciable improvement in the interlaminar shear strength (hereinafter called ILSS for brevity) of the resulting composite material.
It has also been found that the peripheral filaments achieve an adequate ILSS but, at the same time, reduce the strength of the product. This is particularly true with the treatment of tows made up of 10,000 or more filaments.
The above phenomena may be explained as follows. In the electrolytic oxidation, OH.sup.- ions in the electrolyte release electrons at the positive electrode and the oxidation is carried out with the nescent oxygen formed together with water. When a carbon fiber in the form of multi-filament tow is oxidized, it is presumed that the OH.sup.- ions are mostly consumed by the release of electrons to the outer filaments before they reach the center of the tow, and only a minor part of the OH.sup.- ions that have passed out of contact with the outer filaments contribute to the oxidation of the central portion. Hence, uniform surface treatment is impossible, and difficulties are involved in the choice of conditions that would avoid extreme loss of strength while securing ILSS to some extent.
Therefore, it is a principal object of the present invention to provide a process for the surface treatment of carbon fiber through electrolytic oxidation whereby the fiber can be uniformly surface treated.
Another object of the invention is to provide a process for the surface treatment of carbon fiber through electrolytic oxidation whereby uniformity in the degree of surface oxidation is ensured to both the central and peripheral portions of a carbon fiber tow consisting of a number of filaments by adequate supply of OH.sup.- ions to the center of the tow.
Still another object of the invention is to provide a process for the surface treatment of carbon fiber through electrolytic oxidation which can be practiced with ease making use of existing equipment.
Yet another object of the invention is to provide a process for the surface treatment of carbon fiber through electrolytic oxidation whereby a number of carbon fiber tows can be continuously surface treated to attain constant quality.
A further object of the invention is to provide a process for the surface treatment of carbon fiber through electrolytic oxidation whereby the carbon fiber is obtained for the manufacture of high-strength carbon fiber-reinforced composite materials.